Conventionally, while double postcards are used commonly, round trip envelopes are not so common.
Japanese Patent Application Laid-Open No. 10-119975 (Patent Document 1) discloses a round trip envelope which is provided with two sealing flaps of a sealing flap (4) for outgoing mail and a sealing flap (3) for return mail.
Domestic Re-publication of PCT International Publication No. 2003/055759 (Patent Document 2) discloses a round trip envelope which uses a same face of the envelope for displaying the outgoing address and the return address. Sealing of a tongue (5) is broken when opening the envelope. The outgoing address is written on the tongue (5), while the return address is written in that portion on the envelope which is hidden by the tongue (5).
Japanese Patent Application Laid-Open No. 2010-132313 (Patent Document 3) also discloses a round trip envelope which uses a same face of the envelope for displaying the outgoing address and the return address. The return address is written on the back of a flap (22, 23).
Japanese Patent Application Laid-Open No. 1111-49178 (Patent Document 4) discloses a method for making a round trip envelope and a method for sealing. A sheet for writing the outgoing address thereon is removably adhered to a front face of the envelope. After the envelope is opened, the sheet is removed, so that the envelope is used for return mail.
Japanese Utility Model Registration No. 3123468 (Patent Document 5) discloses a round trip envelope which has transparent surfaces and sealing paper with two strips of an adhesive strip 4 for outgoing mail and an adhesive strip 6 for return mail. The envelope uses a same face of the envelope for displaying the outgoing address and the return address.
Japanese Utility Model Registration No. 3044397 (Patent Document 6) discloses a round trip envelope which has a front face on which the outgoing address is written and a back face on which the return address is written.